The invention relates to an extendable and retractable strut and more particularly the invention relates to a lift support strut with a directional damper that includes a movable resilient member that produces damping forces as the resilient member is moved through the strut housing in first and second directions, the damping force being greater when the resilient member is moved in the first direction than when the resilient member is moved in the second direction.
Directional dampers provide direction dependent damping forces that are greater when the movable damping element is displaced in a first direction than when the movable element is displaced in a second direction. Directional dampers used in a strut for controlling the displacement of movable panels such as lift gates on a van or other vehicles provide higher damping forces when the lift gate is opened than when the lift gate is closed. For purposes of the description of the invention and the prior art, generally a xe2x80x9cstrutxe2x80x9d is any device that provides damping and spring forces to control the location and movement of a moveable member.
Such conventional struts include a housing that defines a chamber with a piston member that is movable linearly through the housing chamber as the lift gate or panel is opened and closed. In such devices the housing is charged with a volume of a gas, such as nitrogen, that is pressurized and a liquid such as oil that is flowed from one housing end to the other as the piston is displaced through the housing. As the movable panel is opened and closed and the piston travels through the housing, the strut provides the requisite damping, spring and hold open forces to control the displacement and location of the movable panel.
In conventional strut members, when the door is pulled from the fully closed position a positive force must be applied to the door by the operator in order to open the door, and once the door is displaced from the closed position by an angular distance, typically between 10 and 20 degrees, the spring force is most often sufficient to continue moving the door to the open position without significant supplemental force input by the movable panel operator. During such movement, primary damping is applied to the piston to counteract the spring forces extending the strut. As the door approaches the fully open position, increased second stage damping is applied to the piston to further retard the panel motion and thereby prevent a harmful collision when the strut is fully extended.
Because conventional struts use gas and oil to provide the requisite spring and damping forces in conventional struts, the supplied damping, hold open and spring forces can deviate from their desired values as a result of fluctuations in the ambient temperatures and negatively affect the functionality of the strut. For example, when the ambient temperature is low the gas pressure is reduced, thereby reducing the strut housing pressure and ultimately the hold open and spring forces of the unit. Moreover, because the spring force is low, the spring may be easily compressed permitting the panel to be displaced with a velocity that may result in damage to the panel or strut as the panel reaches its fully closed position. Alternatively, when the ambient temperature is high, the supplied spring forces are increased and as a result, the panel operator must supply greater than usual force to overcome the higher spring forces. The increased spring forces will increase the force required to close the panel making closing the lift gate or panel difficult.
Frequently, such movable panels are displaced by a motor. Such a motor must be large enough to be able to move the specific weight panel and also must be able to overcome spring force. Because the variations in the forces supplied by conventional fluid filled struts are great it is necessary to oversize the motor in order to effectively move the panel during the percentage of the time when the spring forces of the greatest magnitude are present. It would be desirable to utilize a smaller motor to open the lift gate. However a smaller horsepower motor may only be used if the strut is not temperature dependent and susceptible to variations in spring forces.
The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative lift strut with directional damping that is not affected by changes in ambient temperatures. Accordingly, a suitable alternative lift strut with directional damping is provided including features more fully disclosed hereinafter. An even smaller motor may be used if the strut maintains the panel in static equilibrium during its full range of motion.
In one aspect of the present invention this is accomplished by providing a strut including a housing comprising a wall that defines a housing chamber, the housing defining an axis; spring means located in the chamber; and a directional damper comprising piston means movable through the chamber along the axis in a first direction and a second direction, the spring means biasing the piston member in the first direction, the piston means comprising a shaft, a seat connected to the shaft along the shaft length, and a resilient member fixed to the seat, the resilient element comprising at least one primary damping ridge and a damping tongue, each of the at least one primary damping ridges being in friction engagement with the housing as the resilient member is moved in the first and second directions, the damping tongue being in engagement with the housing wall when the resilient member is moved in the first direction and being located away from the housing wall when the resilient member is moved in the second direction.
The foregoing and other aspects will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing figures.